Engine control systems require accurate control of the air/fuel ratio for controlling regulated emissions. To improve the airflow measurement from either a mass air flow based system or a manifold absolute pressure based system, throttle position is often used to produce an additional measurement of airflow. Further because the throttle position sensor leads both a mass airflow sensor and a manifold absolute pressure sensor during a transient, the throttle position is often used as a leading indicator of airflow. In other words, during a transient, a more accurate prediction of airflow into the cylinder can be obtained using the throttle position rather than the conventional mass air flow based sensor or manifold absolute pressure sensor. The throttle position is used to predict the airflow change into the intake manifold that in turns creates an air pressure change in the intake manifold. Using the predicted air pressure allows one to predict airflow into the cylinder.
A problem with using the throttle position as a leading indicator of airflow is the susceptibility to noise. In other words, the lead tends to amplify noise and thus add to air/fuel ratio control error. A particular problem are noise spikes due to factors such as, for example, temporary loss of contact or large changes in the electrical load. These extremely large and extremely short noise spikes are improperly interpreted as large changes in airflow, thus resulting in large fueling errors.
One approach to removing these fueling errors would be to minimize the noise spikes by using a lag filter. This method serves to dampen, or slow down, the system response, thus reducing transient errors. Such a system is described in U.S. Pat. No. 4,958,609.
The inventors herein have recognized numerous disadvantages when trying to apply the above system to reducing the effect of noise spikes. First, when using a first order lag filter, or any other lag filter, a portion of the spike will always pass through the filter. This is because a noise spike contains almost all frequencies and lag filters can only reduce frequencies above a certain frequency known to those skilled in the art as a corner frequency. Because the spike is large relative to the signal, even a small portion of the spike affects the air/fuel control. Second, the decreased system response caused by a lag filter counteracts the lead effect for which the throttle position is used. Third, the more the spike effect is reduced, the slower the system response.